Current-limiting sensors are known, and reference is made to, for example, U.S. application Ser. No. 213,049, filed Dec. 4, 1980, Dietz, now U.S. Pat. No. 4,356,065 assigned to the asignee of the present application, which describes a sensor structure providing an analog output signal representative of oxygen contentration of a gas to which the sensor is exposed. Calibration of such sensors is difficult. It is possible to remove the sensor from the apparatus with which it is associated, for example an exhaust system of an internal combustion (IC) engine and introduce the sensor into a measuring or test apparatus, in which a test program is run which checks if typical values appropriate for the sensor are being maintained. It was not possible to test the sensor when it is in actual operation or association with the engine. If, in due course and due to aging, for example, the output signals from the sensor vary with respect to given gas compositions derived from the engine, exchange of the sensor is required or the associated electrical evaluation circuitry must be recalibrated. Small changes in output signals with a given test gas can usually not be recognized when the sensor is installed in a motor vehicle. Even if the sensor were entirely inoperative, or so severely damaged as not to provide reasonable or expected output signals, a substantial period of time may elapse before it is recognized that the sensor is damaged and should be replaced. In the meanwhile, erroneous engine operation may be commanded by output signals derived from the sensor.